ASTRONOMY. 
follows the apparent course of the sun. 
Some have thought that they could discover 
spots upon its disc; but Herschel has not 
been able to see them ; consequently the 
time of rotation round its axis is not decid- 
edly known. Venus also appears with 
phases, and transits sometimes take place ; 
which are of very great importance in 
astronomy. 
The Earth which vve inhabit is as has been 
proved, a globular body ; it is not, however, 
a perfect sphere, but a spheroid, having its 
equatorial diameter longer than the polar 
diameter or axis. It is consequently flattest 
at the poles, and more protuberant at the 
equator. The diameter at the equator is 
7893 English miles; that at the pole is 7928 
miles. The surface of the earth is much 
diversified with mountains and vallies, land 
and water. The highest mountains in it 
are ’the Andes in South America, some of 
which are about four miles in perpendicular 
altitude. About two-thirds of the globe is 
covered with water. In consequence of the 
earth’s being a globe, people standing upon 
opposite sides of it must have their feet to- 
wards to each other. When in this situation 
they are called antipodes to each other. 
Hence it appears that there is no real up or 
down ; for what is up to one country is 
down to another. It must seem strange to 
those who are ignorant of the shape of the 
the earth, to suppose that if we could bore 
a hole downwards, deep enough, we should 
come to the other side of the world, where 
we should find a surface and sky like our 
own ; yet if we reflect a moment we shall 
perceive that this is perfectly true. As we 
are preserved in our situations by the power 
of attraction which draws us towards the 
centre of the earth, we call that direction 
down which tends to the centre, and the 
contrary. We mentioned before that the 
earth has two motions, the one a diurnal 
motion round its own axis, the other an 
annual motion round the sun. It is the for- 
mer which causes light and darkness, day and 
night ; for when one side of the earth is turned 
towards the sun it receives his rays and is 
illuminated, causing day ; on the contrary, 
when one side of the earth is turned from 
the sun, we are in darkness, and then we 
have night. We see, therefore, by how 
much more simple means this change is 
effected, than they imagined who supposed 
that the earth was fixed, and that the im- 
mense globe of the sun was whirled round 
the earth with the amazing velocity that 
would be necessary. Twilight is owing to 
the refraction of the rays of light by our 
atmosphere through which they pass, and 
which, by bending them, occasion some to 
arrive at a part of the earth that could not 
receive any direct rays from the sun. It is 
the annual motion of the earth round the 
sun which occasions the diversity of seasons. 
To understand this, we must observe what 
has been already mentioned, that the axis of 
the earth is inclined to the plane of its orbit 
2 3|°, and it keeps always parallel to itself ; 
that is, it is always directed to the same 
star. Let fig. 5, Plate II. represent the 
earth in different parts of its elliptic orbit. 
In the spring the circle which separates the 
light from the dark side of the globe called 
the terminator, passes through the poles n,s, 
as appeals in the position A. The earth 
then, in its diurnal rotation about its axis, 
has every part of its surface as long in light 
as in shade ; therefore the days are equal to 
the nights all over the world ; the sun being 
at that time vertical to the equatorial parts 
of the earth. As the earth proceeds in its 
orbit and comes into the position B, the sun 
becomes vertical to those parts of the earth 
under the tropic, and the inhabitants of the 
northern hemisphere will enjoy summer on 
account of the solar rays falling more per- 
pendicularly upon them ; they will also have 
their days longer than then’ nights, in pro- 
portion as they are more distant from the 
equator ; and those within the polar circle, 
as will be perceived by the figure, will have 
constant day-light. At the same time the 
inhabitants of the southern hemisphere have 
winter, their days being shorter than their 
nights, in proportion as they are farther from 
the equator; and the inhabitants of the polar 
regions will have constant night. The earth 
then continues its course to the position C, 
when the terminator again passes through 
the poles, and the days and nights are equal. 
After this the earth advances to the position 
D, at which time the inhabitants of the 
northern hemisphere have winter, and their 
days are shorter than their nights. The 
positions B and D are the solstitial points, 
and A and C the equinoctial points ; they 
are not equidistant from each other, because 
the sun is not in the centre but in the focus 
of the ellipsis. In summer, when the earth 
is at B, the sun is farther from it than in the 
winter when the earth is at D ; and in fact, 
the diameter of the sun appears longer in 
winter than in summer. The difference of 
heat is not owing to the sun’s being nearer 
